Magnetic resonance imaging (magnetic resonance tomography MRT, also MR for short; also called nuclear spin tomography; abbreviated to MRI) is an imaging method used primarily in medical diagnosis for the representation of structure and function of tissue and organs in the human body. It is based on the physical principles of nuclear spin resonance and is therefore sometimes also referred to as nuclear spin tomography.
Magnetic resonance imaging can be used to generate slice images of the human (or animal) body that allow an assessment of the organs and of many pathological organ changes. Magnetic resonance tomography is based on strong magnetic fields and alternating electromagnetic fields in the radio-frequency range that resonantly excite atomic nuclei (usually hydrogen protons). Electrical signals are induced in a receiver circuit. Neither X-ray radiation nor any other ionizing radiation is generated or used in magnetic resonance imaging. The different relaxation times of different types of tissue form an essential basis for the image contrast. In addition, the different content of hydrogen atoms in different tissues (e.g. muscle, bones) also contributes to the image contrast.
In order to obtain an image on the basis of magnetic resonance imaging, that is to say to generate a magnetic resonance recording of an examination object, firstly the body or the body part to be examined of the patient is exposed to a static basic magnetic field, which is as homogeneous as possible and which is generated by a basic field magnet of the magnet system of the magnetic resonance imaging installation. During the recording of the magnetic resonance images, rapidly switched gradient fields for spatial coding are superimposed on this basic magnetic field. The gradient fields are generated by the gradient coils of the magnet system. Moreover, a radio-frequency antenna of the magnetic resonance imaging installation radiates radio-frequency pulses having a defined field strength into the examination volume. For this purpose, the magnetic resonance imaging installation generally includes a fixedly incorporated radio-frequency antenna, the so-called whole body coil. The atoms in the examination object are excited by the radio-frequency pulses in such a way that the atoms are deflected by a so-called “excitation flip angle” from their equilibrium position running parallel to the basic magnetic field. The magnetic resonance signals generated during “deflection back” are detected by at least one non-stationary local coil and fed for further processing. In this case, the local coil is arranged as near to the patient as possible, e.g. placed on the patient.
The basic field magnets are generally superconducting magnets that require a regulatable current supply that makes available the energy for establishing the magnetic field. Electronic and electrical components installed on the basic field magnet of the magnetic resonance imaging installation likewise require a current supply. The use of switching converters for an energy-efficient current supply with little power loss proves to be difficult since multiples of the switching frequencies and the mixed products thereof may cause disturbances in the MR signal.
It is known from the art to carry the current supplies for the superconducting magnets for a service deployment (ramp-up, ramp-down) as a service tool. This causes a logistical complexity and requires thorough preplanning. An immediate or even spontaneous deployment is not possible.
A further possibility for current supply is a fixed installation, which, in a manner similar to the mobile variant, has to be set up outside the region of influence of the magnetic field and also outside the electromagnetic shielding required for magnetic resonance imaging installations (the so-called radio-frequency shielding cabin or RF cabin).
In both embodiments described, the entire magnetization current (sometimes also referred to as “magnet current”), which as a general rule is a few hundred amperes, has to flow via long cables and in some instances through bushing filters.
Current supplies for the electronic and electrical components operated within the electromagnetic shielding are generally installed outside the shielding cabin. Voltage is fed in via a multiplicity of lines respectively requiring bushing filters. Linear regulators or switching converters with corresponding filtering are usually used in the components themselves. The use of ferrite-containing inductive components or (ferro)magnetic components poses a particular challenge since saturation effects, a deformation of the basic magnetic field and considerable force actions may occur.